The electroretinogram (ERG) as a clinical and research tool has been largely limited to use as an indicator of transretinal activity, with little attention being given to the isolation of identifiable response components. However, stimulus and recording techniques are available that can selectively stimulate a localized region of the retina and monitor response that arises therefrom, i.e., the focal ERG. The results of intraretinal studies, and some gross ERG findings, indicate that with appropriate stimulus design a corneal response dominated by the late receptor potential may be recorded. An existing data base derived from primate local ERG studies can be used to differentiate among response components obtained with unconventional stimuli that are required to maximize the late receptor potential. The proposed research is aimed at refining stimulus and monitoring techniques for isolating specific response components (with an emphasis on the late receptor potential), specific retinal regions, or both. After demonstrating an ability to record a response from a specific cell type, we will use the response to study neural processes at various levels within the human retina. These techniques could then be used to probe disease processes, and subsequently to develop new diagnostic tests.